System and method for robust ofdm synchronization

ABSTRACT

Systems and methods are disclosed and include a method that includes adding a training symbol prefix to an OFDM symbol frame, the prefix including a plurality of training symbols, each including N sub-symbol fields. N/2 of the sub-symbol fields are zero valued, and N/2 of the sub-symbol fields carry corresponding symbols of a N/2 sub-symbol pseudo random training symbol. A first half of the pseudo random training symbol is symmetrical to a second half of the pseudo random training symbol. An OFDM N-sub-carrier transmission carries the prefix as signal power on a first N/2 of its N sub-carriers and suppresses signal power on a second N/2 of the sub-carriers. The first N/2 and second N/2 sub-carriers alternate in the frequency domain.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application is a continuation of pending U.S. patent applicationSer. No. 15/723,130, filed on Oct. 2, 2017, and entitled “System andMethod for Robust OFDM Synchronization,” and which is incorporated byreference herein in its entirety.

BACKGROUND

Orthogonal Frequency Division Multiplexing (OFDM) is included in variousterrestrial standards, for example, long-term evolution (LTE) fourthgeneration (4G), and in certain wireless local area network (WLAN)protocols. OFDM generally requires the OFDM receiver to accurately andstably synchronize to OFDM signals that arrive at the receiverantenna(s). Synchronization error can induce loss of orthogonality amongthe OFDM subcarriers, resulting in degradation beyond what would beexperienced by traditional systems. In addition, satellite systems canemploy powerful low density parity check (LDPC) coding, requiringreceivers to operate at low levels of signal-to-noise ratio (SNR), whichcan further complicate the synchronization task.

One synchronization technique, commonly referred to as the “Schmidl andCox” technique (See, T. M. Schmidl and D. C. Cox, “Robust Frequency andTiming Synchronization for OFDM,” IEEE Trans. on Communications, Vol.45, pp. 1613-21, December 1997), provides timing and frequencysynchronization by sending two OFDM training symbols, the firstcontaining identical halves. This technique, however, has technicalshortcomings. For example, the second OFDM training symbol provides forestimation of only the even integer part of the frequency offset. Thislimits the accuracy of offset estimation that can be obtained from thetraining symbols, and also necessitates additional computation. Anothertechnical shortcoming of the Schmidl and Cox is a requirement of highSNR, due to its differential operation.

Another technique for OFDM timing and frequency synchronization, is aclosed-loop method using a feedback loop that includes a post FastFourier Transform (FFT) estimation. The closed loop, though, can induceinstability in some applications.

SUMMARY

This Summary identifies example features and aspects, and is not anexclusive or exhaustive description of the disclosed subject matter.Whether features or aspects are included in, or omitted from thisSummary is not intended as indicative of relative importance of suchfeatures. Additional features and aspects are described, and others willbecome apparent to persons skilled in the art upon reading the followingdetailed description and viewing the drawings that form a part thereof.

An example disclosed system for synchronizing a receiving of anorthogonal frequency division multiplexing (OFDM) can include an inputdata interface configured to receive a sequence signal samples andassociated indexes, a processing unit; and a memory configured to astore a plurality of instructions that when read and executed by theprocessing unit can cause the processing unit to detect a timing metricat each of a succession of the indexes and determine, as a first peak,an index at which the timing metric is at a local maximum, and can causethe processing unit to detect the timing metric at indexes within afirst region of the indexes and determine, as a second peak, an index atwhich the timing metric is at another local maximum, the first regionbeing based in part on the first peak, select a coarse offset as thesmaller of the first peak and the second peak, detect a correlationmetric at each of a plurality of indexes within a second region of theindexes, the second region being based at least in part on the coarseoffset, generate, as an estimated time offset, an index at which thedetected correlation metric is at a local maximum, and apply a timingcorrection, or a frequency correction, or both, to a recovery of symbolsfrom the signal samples, which can be based at least in part on theestimated time offset.

An example disclosed method for communicating a robust synchronizationOFDM frame can include generating an OFDM symbol frame; adding atraining symbol prefix to the OFDM symbol frame, the training symbolprefix including a plurality of training symbols, each of the trainingsymbols including N sub-symbol fields. A first N/2 of the sub-symbolfields can be set at a zero value, a second of the N/2 sub-symbol fieldscan carry corresponding symbols of a N/2 sub-symbol pseudo randomtraining symbol, and a first half of the pseudo random training symbolcan be symmetrical to a second half of the pseudo random trainingsymbol. An example implementation can include transmitting the trainingsymbol prefix as a multi-carrier transmission within an OFDMN-sub-carrier transmission, and the multi-carrier prefix transmissioncan signal power on a first N/2 of the N sub-carriers and can suppresssignal power on a second N/2 of the sub-carriers, and the first N/2sub-carriers and the second N/2 sub-carriers can be aligned atalternating positions in the frequency domain.

An example disclosed method for synchronizing a receiving of an OFDMcommunication can include receiving a sequence of signal samples andassociated indexes; detecting a timing metric at each of a succession ofthe indexes and determining, as a first peak, an index at which thetiming metric is at a local maximum; and can include detecting thetiming metric at indexes within a first region of the indexes anddetermining, as a second peak, an index at which the timing metric is atanother local maximum, the first region being based in part on the firstpeak; selecting a coarse offset as the smaller of the first peak and thesecond peak; detecting a correlation metric at a plurality of indexeswithin a second region of the indexes, the second region being based atleast in part on the coarse offset; determining, as an estimated timeoffset, an index at which the correlation metric is a local maximum; andapplying a timing correction, or a frequency correction, or both, to arecovery of symbols from the signal samples, based at least in part onthe estimated time offset.

BRIEF DESCRIPTION OF THE DRAWINGS

The drawing figures depict one or more implementations in accord withthe present teachings, by way of example only, not by way of limitation.In the figures, like reference numerals refer to the same or similarelements.

FIG. 1 illustrates a functional block diagram of one example zeroed oddfrequency training symbol (ZFS) orthogonal frequency divisionmultiplexing (OFDM) transmission system.

FIG. 2 illustrates a functional block diagram of one example ZFS robustsynchronization OFDM receiver system.

FIG. 3 illustrates a functional block diagram of a flow of operations ina ZFS time and frequency offset estimation process, in a ZFS robustsynchronization process.

FIG. 4 illustrates a functional flow block diagram of operations in oneexample process for accumulated ZFS offset estimation, in another ZFSrobust synchronization process.

FIG. 5 illustrates a functional block diagram of one example ZFS robustsynchronization OFDM receiver with exemplary open loop post-DFTcorrection.

FIG. 6 illustrates a functional flow block diagram of operations in anexample flow for accumulated ZFS offset estimation, in anotheraccumulated correlation ZFS robust synchronization process.

FIG. 7 illustrates a functional block diagram of one example enhancedZFS robust synchronization OFDM receiver.

FIG. 8 illustrates a simplified functional block diagram of one exampleZFS robust synchronization OFDM satellite communication system.

FIG. 9 illustrates a functional block diagram of an example computersystem upon which aspects of this disclosure may be implemented.

DETAILED DESCRIPTION

In the following detailed description, numerous specific details are setforth by way of examples in order to provide a thorough understanding ofthe relevant teachings. However, it should be apparent that the presentteachings may be practiced without such details. In other instances,well known methods, procedures, components, and/or circuitry have beendescribed at a relatively high-level, without detail, to avoidunnecessarily obscuring aspects of the present teachings.

FIG. 1 illustrates a functional block diagram of one example zeroed oddfrequency training symbol (ZFS) orthogonal frequency divisionmultiplexing (OFDM) transmission system 100, according to one or moreaspects. It will be understood that the abbreviation “ZFS,” as usedherein, is a coined abbreviation carrying no meaning other than: “zeroedodd frequency training symbol.” The system 100 is configured to transmitan N-sub-carrier OFDM signal, including N sub-carriers that are mutuallyorthogonal, with each sub-carrier modulated to carry a series ofsub-symbols, that in combination can form a series of N-OFDM symbols,one from each of the sub-carriers. The system 100 can be configured toformat the OFDM signal as frames, each including M OFDM symbols, and toinsert ZFS training symbols between the frames. In an aspect, an ZFSrobust synchronization OFDM receiver system, described in greater detailin later sections, can apply particular processes that can rapidlyestimate timing and frequency offset at the receiver, through particularutilizations of features of the ZFS training symbols.

Referring again to FIG. 1, the ZFS OFDM transmission system 100 caninclude an encoder and interleaver 102, configured to receive an inputinformation signal stream, SP, and output a suitably encoded andinterleaved information stream, LP, to a constellation mapper 102. Theparticular operations of the encoder and interleaver 102 can beaccording to techniques not necessarily specific to the disclosedsystems and methods, and therefore further detailed description of suchtechniques is omitted.

The constellation mapper 104 can map the information symbols to an M-arysignal space in which the symbols are, for example, X-Y positions in acomplex plane, with “X” being an in-phase component and “Y” being a “j”or quadrature component. The particular operations performed in themapping can be according to techniques not necessarily specific to thedisclosed systems and methods, and therefore further detaileddescription of such techniques is omitted. For purposes of avoidingunnecessary description not relevant to disclosed concepts, theconstellation mapper 104 will be assumed as configured for a four-stateQAM signal space of C+Dj, where C can be +1 or −1, and D can be +1 or−1, or scaled versions thereof. This is only an example, though, and isnot intended as a statement of preference or to be any limitation on therange of implementations according to this disclosure.

The ZFS training symbol insertion unit 106 can be configured to insertthe ZFS training symbols between OFDM successive frames. The ZFStraining symbols can include a first ZFS training symbol and a secondZFS training symbol in immediate succession, i.e., as two OFDM symbolslots, between successive frames. The ZFS training symbol insertion unit106 can be implemented, for example, using storage registers (notvisible in FIG. 1), or locations in a memory resource of a generalpurpose programmable computer (not visible in FIG. 1), or otherprogrammable processing engine (not visible in FIG. 1). Anotherimplementation of the ZFS training symbol insertion unit 106 can provideor support a process for generating the ZFS training symbols. Forexample, the ZFS training symbol can be generated by performing aninverse Fast Fourier Transform (IFFT) of one-half size and thenrepeating its output in the time domain.

The serial/parallel unit 108 can be configured to feed successive OFDMsymbols, including ZFS training symbols followed by frame symbols, assuccessive blocks of N QAM coded sub-symbols into an N-point inverseDiscrete Fourier Transform (IDFT) unit 110. The N-point IDFT unit 110can generate, for each sub-symbol, a time domain sample of acorresponding one of N sub-carriers, each effectively modulated inamplitude and phase, to one of four amplitude-phase positions, by thevalues of C and D. The N-point IDFT unit 110 can be implemented, forexample, as an N-point IFFT device. Particular operations performed bythe N-point IDFT unit 110, whether implemented as an IFFT or otherwise,can be according to techniques not necessarily specific to the disclosedsystems and methods, and therefore further detailed description of suchtechniques is omitted.

Referring again to FIG. 1, the ZFS OFDM transmission system 100 can alsoinclude a parallel to serial unit 112 that can re-form the parallel Ntime domain outputs of the IFFT unit 110 into a serial block or windowof N sub-symbol time samples. The serial block of N sub-symbol timesamples can be input to a pulse shaping filter 114, the output of whichcan be transmitted over a radio channel 116. The radio channel 116 caninclude a radio frequency (RF) upconverter (not visible in FIG. 1) thatcan frequency shift the filtered N sub-symbols to an RF band for radiotransmission from a transmission antenna (not visible in FIG. 1). Thespecific frequency(ies) can be application specific. The radio channel116 can also include a propagation path (not visible in FIG. 1) from thetransmission antenna to a ZFS robust synchronization receiver (notvisible in FIG. 1) that can be according to one or more systems andmethods for ZFS robust synchronization receiving disclosed herein.

As described above, the ZFS training symbol insertion unit 106 can beconfigured to insert ZFS training symbols between OFDM informationframes. Referring to Table I below, various features and aspects of theZFS training symbols will be described. These features and aspects, incombination with particular receiver processes described in greaterdetail later, can provide rapid and accurate estimation of timing offsetand frequency offset at the receiver, at signal-to-noise-ratios (SNRs)that can be lower than SNRs at which conventional offset estimationtechniques can provide acceptable operation. The ZFS training symbolsshown in Table 1 are examples, for purposes of illustrating certainconcepts and various aspects thereof, and are not intended as anylimitation on the scope of practices or implementations according tosuch concepts and aspects.

TABLE 1 Subcarrier Index k C_(1,v) C_(2,v) 0 1 + 1j 1 − 1j 1 0 0 2 −1 +1j 1 + 1j 3 0 0 . . . . . . . . . N − 2 1 + 1j 1 − 1j N − 1 0 0

Referring to Table 1, each of the N rows corresponds to a “k”sub-carrier index that ranges, in single step increments, from “0” up toN−1, “N” being an integer. For purposes of this description,sub-carriers at k=0, 2, 4 . . . , N−2, can be referred to as “evensub-carriers” or “even frequencies,” and sub-carriers at k=1, 3, 5 . . ., N−1, can be referred to as “odd sub-carriers” or “odd frequencies.” Itwill be understood that the naming convention as which of the sets ofN/2 sub-carriers is “even” and which is “odd” labels “even” and “odd” isarbitrary. As illustrated in Table 1, ZFS training symbols can includeN/2 sub-symbols interleaved with zeroes. The N/2 sub-symbols areconfigured to form each ZFS training symbols as a pseudorandom number,as described in greater detail later. Depending on whether thealternating zeros begin at the first sub-carrier (k=0) or the secondsub-carrier (k=1), and depending on the even-odd naming convention used,OFDM transmission of ZFS training symbols sets either the evensub-carriers or the odd sub-carriers at zero. For brevity, the remainderof this description arbitrarily configures the zero pattern in themanner illustrated in Table 1 which, using the described even-odd namingscheme, sets the odd sub-carriers at zero. It will be understood thatdisclosed systems and methods can be alternatively implemented with ZFStraining symbol configurations that set the even sub-carriers at zero.

Referring again to Table 1, the example ZFS training symbols include afirst ZFS training symbol, C1,v and a second ZFS training symbol C2,v.The index “v” is a frame index, corresponding to the OFDM informationframe that immediately follows the C1,v and C2,v.

The Table 1 first ZFS training symbol C1,v and the second ZFS trainingsymbol C2,v each contain a pseudorandom (PN) sequence of sub-symbols atrow positions corresponding to even sub-carriers and contain zeroes atrow positions corresponding to odd sub-carriers. The Table 1 example PNsequence carried by the first ZFS training symbol C1,v is: +1+1j; −1+1j,. . . +1+1j, and the example PN sequence carried by the second ZFStraining symbol C2,v is: +1−1j; 1+1j, . . . +1−1j. In an aspect, thefirst ZFS training symbol C1,v, and each of the successive first secondZFS training symbols, e.g., C1,v+1; C1,v+2, . . . (not visible inTable 1) have a symmetry in which the sub-sequence of N/2 sub-symbols(of which N/4 are zero) forming the first half of the PN sequence isidentical to the N/2 sub-symbols (of which N/4 are zero) forming thesecond half of the PN sequence. ZFS time offset and frequency offsetestimation techniques applied at ZFS receivers described in greaterlater, can exploit this symmetry of the two ZFS training symbols.

FIG. 2 shows a functional block diagram of one example ZFS/robustsynchronization OFDM receiver system 200, (hereinafter “system 200”).The system 200 can include an analog-to-digital converter (ADC) 202formed, for example, of an in-phase ADC 202A for sampling an in-phaseSignal In, labeled SR(t), and a quadrature in-phase ADC 202B, labeledSJ(t), for sampling a quadrature Signal In. SR(t) and SJ(t) can beprovided, for example, by an in-phase down-converter and a quadraturedownconverter of an RF receiver (not visible in FIG. 2).

ADC 202A and 202B respectively output OFDM signal samples SR(d) andSJ(d), collectively “S(d).” The signal samples S(d) can be input to theZFS OFDM robust synchronization logic 204. As described above inreference to FIG. 1, ZFS training symbols are inserted between frames.The illustrated ZFS OFDM robust synchronization unit 204 is configuredto estimate time offset and frequency offset of the system 200 based onthe ZFS training symbols and, to apply particular offset correctionprocesses as will be described in greater detail. The ZFS OFDM robustsynchronization unit 204 can include a correlation buffer 206,configured to receive a stream of samples S(d) and store, for latercross-correlation and auto-correlation operations, a succession ofapproximately 2N of the samples. It will be understood that the term“buffer,” as used herein in the context “of correlation buffer” 206,means a logic function for which implementation is not limited to anyparticular hardware or software configuration or architecture. Oneimplementation of the correlation buffer 206 can include an array ofregisters (not separately visible in FIG. 2), arranged in series—for aserial-in-serial-out clocking of the S(d) samples—in addition to beingindividually addressable. The correlation buffer 206 can be configuredwith particular accessibility for enabling cross-correlationcalculations, e.g., capability of fast access to pairs of samples S(d)in the correlation buffer 206, spaced apart by cross-correlation delays,as will be described in greater detail.

The ZFS OFDM robust synchronization logic 204 can further include acorrelation engine 208, configured with capability of computingparticular autocorrelations and cross-correlations of S(d) in thecorrelation buffer 206, over given ranges of “d.” The ZFS OFDM robustsynchronization logic 204 can provide the correlation engine 208 withaccess capability, as illustrated by arrows in FIG. 2, to samples S(d)in the correlation buffer 206 when performing cross-correlation andautocorrelation computations. The ZFS OFDM robust synchronization logic204 can also include a timing and correlation metric engine 210,configured with capability of computing particular metrics, based onautocorrelation data and cross-correlation data generated by thecorrelation engine 208.

It will be understood that “engine,” as used herein in the context of“correlation engine” 208 and “timing and correlation metric engine” 210,means a computation capability, and is not limited to any particularhardware or software architecture, or combination thereof. For example,one implementation of the ZFS OFDM robust synchronization logic 204 caninclude an arithmetic logic unit (ALU) (not separately visible in FIG.2), an associated computation controller (not separately visible in FIG.2), providing computational resources sufficient to perform according tothe correlation engine 208 and the timing and correlation metric engine210.

In an aspect, the correlation engine 208 and the timing and correlationmetric engine 210 can be configured with capability to performcomputations of a particular timing metric, M(d), according to thefollowing Equations (1)-(3):

$\begin{matrix}{{M(d)} = \frac{{{P(d)}}^{2}}{\left( {R(d)} \right)^{2}}} & {{Equation}\mspace{14mu} (1)} \\{{{P(d)} = {\sum\limits_{m = 0}^{\frac{N}{2} - 1}{\gamma_{d + m}^{*}\gamma_{d + m + {N/2}}}}},} & {{Equation}\mspace{14mu} (2)} \\{{P(d)} = {\sum\limits_{m = 0}^{\frac{N}{2} - 1}{\gamma_{d + m + {N/2}}}^{2}}} & {{Equation}\mspace{14mu} (3)}\end{matrix}$

where,

m is a cross-correlation and autocorrelation index,

N/2 is one-half the length of the ZFS training samples,

γ_(x) is a signal sample at index position x, e.g., S(d+m+N/2), and

γ_(x)* is a conjugate of the signal sample at index position x.

Configurations to perform computations according to Equations (1)-(3)can include configuring correlation engine 208 to compute P(d) and R(d),for each index d in the range defined above, in accordance withEquations (2)-(3), and provide the resulting P(d) and R(d) to thecorrelation and timing metric engine 210. The timing and correlationmetric engine 210 can be configured to compute, in turn, the magnitudesquared of P(d), and the magnitude squared of R(d), compute the ratio ofthe two magnitudes squared, and output a corresponding M(d).

In an aspect, the correlation engine 208 and the timing and correlationmetric engine 210 can be configured to accumulate correlation data overmultiple frames, and to generate M(d) as an accumulated M(d). Forexample, in one implementation, the correlation engine 208 and thetiming and correlation metric engine 210 can be configured to compute acoherent accumulated timing metric, M_(coh) ^(NF) (d), over NF frames,as illustrated by the following Equation (4), where “coherent” meansphase information is carried in the accumulation:

$\begin{matrix}{{M_{coh}^{NF}(d)} = \frac{{{\sum\limits_{i = 1}^{NF}{P^{(i)}(d)}}}^{2}}{\left( {\sum\limits_{i = 1}^{NF}{R^{(i)}(d)}} \right)^{2}}} & {{Equation}\mspace{14mu} (4)}\end{matrix}$

where

-   -   M_(coh) ^(NF) (d) is the accumulated timing metric, over NF        frames,    -   P_(d) ^((i)) is the Equation (2) cross-correlation metric, at        the ith frame, and    -   R_(d) ^((i)) is the Equation (3) autocorrelation metric, at the        ith frame.

In another implementation, the correlation engine 208 and the timing andcorrelation metric engine 210 can be configured to compute non-coherentaccumulated timing metric, M_(nom) ^(NF)(d), over NF frames, asillustrated by the following Equation (5), where “non-coherent” meansphase information is not carried in the accumulation:

$\begin{matrix}{{M_{non}^{NF}(d)} = \frac{\sum\limits_{i = 1}^{NF}{{P^{(i)}(d)}}^{2}}{\sum\limits_{i = 1}^{NF}\left( {R^{(i)}(d)} \right)^{2}}} & {{Equation}\mspace{14mu} (5)}\end{matrix}$

Technical features of coherent and non-coherent combining ofsynchronization parameters, for example, as defined by Equations(4)-(5), can include improved synchronization performance at lowerlevels of SNR, through accumulation of useful correlation contributionsfrom multiple frames. Coherent accumulation combines both amplitude andphase information of the individual components. Non-coherentaccumulation, in contrast, uses only amplitude information of theindividual components. Non-coherent accumulation can be more robust tofast varying channel conditions, but coherent accumulation can providebetter performance, subject to channel variation rate constraints. Bothcoherent and non-coherent accumulation, though, can provide improvementover correlation based on single-frame computation.

Referring to the FIG. 2 ZFS OFDM synchronization logic 204, forcomputations according to Equation (4), Equation (5), or both, across-correlation accumulator 212 can be provided, configured toaccumulate P_(d) ^(i), for a given (d), over NF frames, and anautocorrelation accumulator 214 can be provided, configured toaccumulate R_(d) ^(i), for a given (d), over NF frames. Their respectiveoutputs can be provided to the correlation and timing metric engine 210.In one example implementation, the cross-correlation accumulator 212 andautocorrelation accumulator 214 can be integrated into the correlationengine 208. In another example implementation, the cross-correlationaccumulator 212 and autocorrelation accumulator 214 can be integratedinto the timing and correlation metric engine 210. In anotherimplementation, the cross-correlation accumulator 212, or theautocorrelation accumulator 214, or both, can be performed, in whole orin part, by computation and memory resources of a shared programmableprocessor.

In one implementation, ZFS OFDM synchronization logic 204 can include apeak detection buffer 216 and associated peak detection logic 218. Thepeak detection buffer 216 can be configured to hold a plurality, e.g., awindow, of timing metric values, e.g., a plurality of M(d) computationsoutput by the timing and correlation metric engine 210. The peakdetection logic 218 can be configured to detect local peaks among theplurality held in the buffer 216. In one implementation, the peakdetection buffer 216, or the associated peak detection logic 218, orboth, can be incorporated into the timing and correlation metric logic210. In another implementation, described functionality of the peakdetection buffer 216, or the associated peak detection logic 218, orboth, can be provided, in whole or in part, by computation and memoryresources of a shared programmable processor.

In an aspect, the ZFS OFDM synchronization logic 204 can include anestimated time offset (ETO) engine 220, configured to generate ETO basedon identifying a peak value of a correlation metric, CM, such as theexample defined by Equation (6) below, over a range of d that can be setaccording to the above-described CTO, as follows:

CM(d)=(|P(d)|² +|P(d+N)|²)/((R(d))²+(R(d+N))²)  Equation (6),

-   -   the range of d being, for example,

${{d - {CTO}}} \leq \frac{N}{2}$

In an implementation, the ETO engine 220 can be configured to generateETO based on a peak value of a coherent accumulated correlation metric,CM_(coh) (d), such as defined by Equation (7) below:

$\begin{matrix}{{{CM}_{coh}(d)} = \frac{{{\sum\limits_{i = 1}^{NF}\left( {{P^{(i)}(d)} + {P^{(i)}\left( {d + N} \right)}} \right)}}^{2}}{\left( {\sum\limits_{i = 1}^{NF}\left( {{R^{(i)}(d)} + {R^{(i)}\left( {d + N} \right)}} \right)} \right)^{2}}} & {{Equation}\mspace{14mu} (7)}\end{matrix}$

In addition, the ETO engine 220 can be configured to generate ETO basedon a peak value of a non-coherent accumulated correlation metric,CM_(non) (d), such as defined by Equation (8) below:

$\begin{matrix}{{{CM}_{non}(d)} = \frac{\sum\limits_{i = 1}^{NF}{{{P^{(i)}(d)} + {P^{(i)}\left( {d + N} \right)}}}^{2}}{\sum\limits_{i = 1}^{NF}\left( {{R^{(i)}(d)} + {R^{(i)}\left( {d + N} \right)}} \right)^{2}}} & {{Equation}\mspace{14mu} (8)}\end{matrix}$

In an implementation, the ETO engine 220 can be configured to generate aplurality of CM(d) values according to Equation (6), for example, overthe range of d defined above for Equation (6), and to generate ETO asthe index d having the peak CM(d) value. Such operations can beaccording to Equation (9) below:

$\begin{matrix}{{{ETO} = {{Max}\left( {{CM}(d)} \right)}},{{{for}\mspace{14mu} {{d - {CTO}}}} \leq \frac{N}{2}}} & {{Equation}\mspace{14mu} (9)}\end{matrix}$

The ETO engine 220 can be additionally configured to generate aplurality of accumulated CM_(coh) (d) values according to Equation (7),for example, over the same range of d, and then generate ETO as theindex d having the peak CM_(coh)(d) value, according to Equation (10)below:

$\begin{matrix}{{{ETO} = {{Max}\left( {{CM}_{coh}(d)} \right)}},{{{for}\mspace{14mu} {{d - {CTO}}}} \leq \frac{N}{2}}} & {{Equation}\mspace{14mu} (10)}\end{matrix}$

The ETO engine 220 can be further or alternatively configured togenerate a plurality of CM_(non)(d) values according to Equation (8),for example, over the same range of d, and generate ETO as the index dhaving the peak CM_(non)(d) value, according to Equation (11) below:

$\begin{matrix}{{{ETO} = {{Max}\left( {{CM}_{non}(d)} \right)}},{{{for}\mspace{14mu} {{d - {CTO}}}} \leq \frac{N}{2}}} & {{Equation}\mspace{14mu} (11)}\end{matrix}$

The ETO engine 220 can also be configured to select between ETO based oncoherent multi-frame accumulation, as per Equation (10) and non-coherentmulti-frame accumulation, as per Equation (11) based, for example, onSNR conditions.

In an implementation, the ETO engine 220, for one or more of Equations(9)-(11), can be mode of the correlation engine 208 operating incombination with the peak detection buffer 216 and peak detection logic218. In another implementation, the ETO engine 220 can include duplicateinstances of one or more of the correlation engine 208, the peakdetection buffer 216 and the peak detection logic 218. In anotherimplementation, the ETO engine 220 can be provided, in whole or in part,by computation resources of a shared programmable processor.

Referring to FIG. 2, the ZFS OFDM synchronization logic 204 can includea frequency offset engine 222 configured to generate an estimatedfrequency offset (EFF), based at least in part on ETO. In oneimplementation, the frequency offset engine 222 can be configured togenerate EFF according to Equation (12) below:

EFF=angle(P(ETO)+P(ETO+N))/π  Equation (12)

In addition, the frequency offset engine 222 can be configured togenerate an accumulated EFF, for example, a coherent accumulated EFFaccording to Equation (13) below:

EFF _(coh)=angle(Σ_(i=1) ^(NF) P ^((i))(ETO)+Σ_(i=1) ^(NF) P^((i))(ETO+N))/π  Equation (13)

In an aspect, a time offset value “TO” can be provided, or obtained,through means not necessarily according to Equation (9), whereupon EFFcan be generated by operations having a form according to Equation (12),using the provided or obtained time offset TO as opposed to ETO. Asanother example, a sequence of NF of such time offsets TO can beprovided or obtained, for example, one of such TO values per frame,whereupon operations according to Equation (13) can be applied to obtainan accumulated EFF, using the sequence of NF provided or obtained timeoffsets TO as opposed to a sequence of NF ETOs as described in referenceto Equation (9).

The ZFS OFDM synchronization logic 204 can also include an offsetcorrection logic 224 that, based on a single frame ETO or an accumulatedETO, can apply time offset correction to the samples S(d) or, based on asingle frame or an accumulated EFF, can apply frequency offsetcorrection to the samples S(d), or can apply both time and frequencyoffset correction, for input to the N-point DFT 226.

Referring again to FIG. 2, time-offset and frequency offset correctedsignal samples can be input to an N-point DFT unit 226 as successivewindows of N samples. The N-point DFT unit 226 can be, but is notnecessarily, configured as an N-point FFT unit. Each of the Ncomplex-valued frequency domain outputs N of the N-point DFT unit 226has magnitude and phase information corresponding to the constellationposition of a corresponding one of the outputs from FIG. 1 constellationmapper 104. The N outputs of the N-point DFT unit 226 can be input to adecoder 228. The decoder 228 accordingly can output an m-bitinformation, with “m” being the number of bits encoded by theconstellation position. The parallel-to-serial buffer 230 can serializethe N m-bit information into a recovered information data RI.

FIG. 3 illustrates a functional block diagram of a flow 300 ofoperations in a ZFS time and frequency offset estimation process of anexemplary ZFS robust synchronization. Operations in the flow 300 aredescribed in sequential order. This is not intended as a limitation onthe temporal relation of operations. On the contrary, various operationsdescribed in reference to different FIG. 3 blocks can be performedconcurrent with one another, or fully or partially overlapping in timewith one another. It will also be understood that the order ofoperations in the flow 300 can be re-arranged from the graphicalarrangement in which they appear on FIG. 3, and from the order in whichthey are referenced in the description.

Referring to FIG. 3, the flow 300 can start at 302 with operations ofreceiving a stream of samples S(d) into a correlation buffer, orequivalent, for purposes of determining particular correlations. Oneexample can be receiving samples S(d) into the FIG. 2 correlation buffer206, which is capable of holding approximately 2N of the samples, andenabling parallel access for cross-correlation and auto-correlationoperations, as described above. The flow 300 can then proceed to 304,and apply operations for detecting a timing metric at each of asuccession of the sample indexes d, and determining, as a first peak, anindex at which the timing metric is at a local maximum. The timingmetric can be, for example, a correlation based timing metric, such asM(d) of Equation (1). Operations at 304 for detecting the timing metric,e.g., M(d), can be performed, for example, by the cross-correlationengine 210 and the autocorrelation engine 212 providing their respectiveoutputs to the timing metric engine 214. Operations at 304 can alsoinclude, for example, storing a plurality of the timing metric M(d)values in a peak detection cache or register of a peak detection engine,e.g., the peak detection cache 218, and searching for a local maximumwithin that cache or register and, upon finding the local maximum,generating its index as a “first peak.” The searching and first peakdetection operations can be performed, for example, using the FIG. 2peak detection engine 220.

The flow 300 can then proceed to 306, and apply operations for detectinga second peak of the timing metric within a sub-region, e.g., “firstregion” of the correlation buffer 206. Such operations can be performed,for example, using the correlation buffer 206, correlation engine 208,timing and correlation metric engine 210, peak detection buffer 216, andpeak detection engine 218 as described above for detecting the firstpeak. Upon detecting the second peak of the timing metric, the flow 300can proceed to 308 and apply operations of selecting or setting, as a“coarse time estimate” (CTO) of the frame start, the smaller of theindex d of the first peak and the index d of the second peak.

Upon setting the coarse time estimate CTO, the flow 300 can proceed to310 and apply operations of detecting a correlation metric, CM, at eachof a plurality of indexes d within a second region of the samples S(d),the second region being based at least in part on the CTO. Operations at310 can include, for each index d within the second region, generating across-correlation data, based at least in part on a correlation windowaligned with the index, and generating an autocorrelation data, based atleast in part on a region of the correlation window, then determiningthe CM based at least in part on a ratio of the cross-correlation datato the autocorrelation data. One example of generating a correlationmetric at 310 can be according to Equation (6) above.

Associated with operations at 310, the flow 300 can apply operations at312 of searching for a local maximum in the correlation metric CM and,upon detecting the local maximum can generate the index d of that localmaximum as the estimated time offset (ETO).

Referring again to FIG. 3, upon determining the ETO at 312, the flow canproceed to 314 and apply operations of generating an estimated frequencyoffset (EFF), based at least in part on ETO. The FIG. 2 frequency offsetengine 222 can be one example implementation of such operations.Specific operations at 314 can include detecting a cross-correlation ofsignal samples S(d) within a region of the time indexes aligned with theETO identified at 312, and generating a corresponding first time offsetcross-correlation data. Operations at 314 can also include detecting across-correlation of signal samples within a region spaced N timeindexes from the region aligned with the ETO, and generating acorresponding second time offset cross-correlation data. Operation at314 can also include generating, as a cross-correlation sum, a sum ofthe first time offset cross-correlation data and the second time offsetcross-correlation data, and determining the EFF based, at least in part,on a scaling of the cross-correlation sum. One example of determiningEFF at 314 can be according to Equation (9) described above.

The flow 300 can proceed from 314 to 316 and apply operations ofcorrecting time offset and frequency offset based, at least in part, onthe ETO generated at 312 and the EFF generated at 314.

In applications where the operating SNR is at a low level, analternative implementation can utilize the FIG. 3 operations fordetecting the CTO, and then apply a coherent combination, oraccumulation of correlations from ZFS training symbols over multipleframes. Estimated time offset ETO can then be generated, for example, byapplying FIG. 3 operations at 312 to the accumulated correlationinformation, instead of only correlation information from a singleframe's ZFS training symbols. Technical features can include an increasein accuracy of the ETO and of the EFF, and corresponding capability ofproviding acceptable OFDM reception and decoding at lower SNR levelsthan conventional offset estimation techniques.

FIG. 4 shows a simplified diagram of a transmission stream 400 that canbe used in an accumulated, multi-frame ZFS training symbol correlation,and estimation of time and frequency offset. As shown, there can be atotal NF data frames and each data frame can have a ZFS training symbolsequence appended to its beginning.

FIG. 5 illustrates a functional flow block diagram of operations in oneexample flow 500 for accumulated ZFS offset estimation, in another ZFSrobust synchronization process. The flow 500 can incorporate the FIG. 3operations of detecting the coarse time offset (CTO), and can then applyan accumulating correlation process which, as described above, canimprove synchronization accuracy, particularly in low SNR environments.

Referring to FIG. 5, an example instance of the flow 500 will bedescribed starting with an initial detection of the ZFS trainingsymbols. More specifically, the flow 500 can start at 502, withoperations of sampling a received signal in. One example of operationsat 502 can include the FIG. 2 ADC 202 sampling a quadraturedown-converted ZFS OFDM transmission such as described in reference toFIG. 1. The sampling outputs can be, for example, the above-describedS(d). Operations at 502 can also include buffering a window of thesamples S(d) using, for example, resources such as the FIG. 2correlation buffer 206. The flow 500 can then proceed to 504, whereoperations of detecting an approximate frame start can be applied. Suchoperations can include, for example, the operations applied in the FIG.3 flow 300 at 304, 306, and 308, namely, detecting a first peak of thetiming metric, detecting a second peak, and setting a coarse time offsetCTO according to the smaller of the two peaks' respective indexes. Theflow 500 can then proceed to 506, where operations such as described inreference to FIG. 3 at 310 and 312 can be applied to generate an initialinstance of the estimated time offset ETO. The initial ETO can be basedon finding a local maximum of the correlation metric CM, for indexpositions of S(d) associated with the CTO determined at 504.

Referring to FIG. 5, after generating the initial ETO at 506, the flow500 can proceed to 508 and initialize an accumulated cross-correlationdata (CCD) and an accumulated autocorrelation data, using a correlationwindow or windows relative to that ETO. Referring to Equation (4) and toFIG. 2, example operations at 508 can include initializing thecross-correlation accumulator 212 with a first iteration of the sum ofproducts numerator of Equation (4) (or of Equation (5) for anon-coherent correlation accumulation), and initializing theautocorrelation accumulator 214 with a first iteration of the sum ofproducts denominator of Equation (4) (or of Equation (5) for anon-coherent correlation accumulation).

After the initializations at 508, the flow 500 can proceed to 510 andreceive a next block or window of samples S(d), e.g., from a continuingoperation of the sampling at 502, and can then proceed to 512, whereoperations can be applied to generate another CCD and another ACD, andcorrespondingly update the running accumulation of each. Operations at512 can apply the correlation windows used at 508, relative to the newlyreceived window of samples S(d). The flow 500 can then proceed to 514,increment the frame counter index i to i+1, and then proceed to a looptermination test at 516, namely whether the frame counter has reachedNF. If the termination condition is not met, the flow 500 can return to510, receive another window of samples S(d), then repeat the 512generation of CCD and ACD and updating the accumulated CCD and ACD, andincrementing the frame count at 514, until the frame count i reaches NF.Upon reaching NF, the flow 500 can proceed to 518 and generate anaccumulated estimated time offset, based on a ratio of the accumulatedCCD to the accumulated ACD. Referring to FIG. 2 and Equation (4) (orEquation (5) for a non-coherent correlation accumulation), suchoperations can be implemented by shifting successive windows of samplesinto the correlation buffer 206 and, for each of the windows, applyingcorrelation engine 208 updating the respective cross-correlationaccumulator 212 and autocorrelation accumulator 214.

FIG. 6 illustrates a functional flow block diagram of operations in anexample flow 600 for accumulated ZFS offset estimation, in anotheraccumulated correlation ZFS robust synchronization process. The flow 600can incorporate the FIG. 3 operations of detecting the CTO). Referringto FIG. 6, an example instance of the flow 600 can start at 602, withoperations of sampling a received signal in, for example, the FIG. 2 ADC202 sampling a quadrature down-converted ZFS OFDM transmission such asdescribed in reference to FIG. 1. Operations at 602 can also includebuffering a window of the samples S(d) using, for example, resourcessuch as the FIG. 2 correlation buffer 206. The flow 600 can then proceedto 604, where operations of detecting an approximate frame start can beapplied. Such operations can include, for example, the operationsapplied in the FIG. 3 flow 300 at 304, 306, and 308, namely, detecting afirst peak of the timing metric, detecting a second peak, and setting acoarse time offset CTO according to the smaller of the two peaks'respective indexes.

Referring to FIG. 6, after detecting CTO at 604, the flow 600 canproceed to 606 and apply operations of initializing the frame index i tozero, and initializing the Equation (7) accumulated CM_(coh)(d) to zero,or initializing the Equation (8) accumulated CM_(non)(d) to zero,depending on whether coherent or non-coherent accumulation is beingapplied, for all d in the range relative to N/2 and CTO specified inEquations (7) and (8). After the initialization at 606, the flow 600 canproceed to 608 and receive, e.g., from a continuing operation of thesampling at 602, a next block or window of samples S(d), for the currenti^(th) frame, and then calculate a first frame iteration, applyingeither Equation (7) or (8), for all d in the specified range. The flow600 can then proceed to 610 and update the accumulated CM(d), over therange of d relative to N/2 and CTO, using the i^(th) frame CM(d)calculated at 608. Upon updating the accumulated CM(d) at 610, the flow600 can proceed to 612, increment the frame index i by one, and thenproceed to the frame count loop escape at 614. In the FIG. 6 illustratedimplementation, the frame index i is incremented after an iteration of608 and 610, and therefore the loop escape can be set at i=NF+1.

Referring to FIG. 6, and to Equations (7) and (8), when theabove-described block 614 escape condition is met, the summation iscomplete for whichever of Equations (7) and (8) is applied, and theresult is a plurality of accumulated correlation metrics CM(d)—one foreach d in the defined range relative to COT and N/2—each of the metricsbeing a coherently or non-coherently combined correlation over NFframes. The flow 600 can then proceed to 616 and determine, as theaccumulated ETO, the maximum of the accumulated correlation metricsCM(d), applying Equation (10) if a coherent accumulation is selected andapplying Equation (11) if a non-coherent accumulation is selected.

FIG. 7 illustrates a functional block diagram of one example enhancedZFS robust synchronization OFDM receiver 700 (abbreviated for brevity as“the receiver 700”) with open loop post-DFT correction, and otherfeatures. The illustrated receiver 700 can include an enhancedZFS/robust synchronization OFDM receiver 702, which can include the FIG.2 ZFS/robust synchronization OFDM receiver system 200, configured toapply coherent combination, or accumulation of correlations from ZFStraining symbols over multiple frames, such as described above inreference to Equations (7) and (8). The receiver 700 can include afrequency correction unit 704, DFT unit 706 and a post DFT estimationunit 708. In an example operation, after time-domain synchronization,for example applying the FIG. 3 flow 300, by single-frame operation ofthe enhanced ZFS/robust synchronization OFDM receiver 702 the receivedtraining symbols are frequency corrected by the frequency correctionunit 704, and then demodulated using the DFT 706. The demodulatedsymbols (in the frequency domain) can then be correlated, by the postDFT estimation unit 708, with known frequency domain training symbols,to generate an OFDM symbol timing estimate.

Operations associated with the post DFT estimation unit 708 can includecoherent time-domain combining N correlated values within each OFDMsymbol and non-coherent time-domain combining over NS (e.g., two ormore) OFDM training symbols in each training sequence and, additionally,accumulating over NF frames (e.g., as described in reference toEquations (7) and (8)). The receiver 700 can be configured to apply apeak searching algorithm to the correlated data to generate an enhancedaccuracy estimate of OFDM symbol timing. The receiver 700 can beconfigured to use that timing estimate to correct any remaining timingoffset, and frequency domain training symbols can be recalculated basedon the updated estimate of the start of FFT window. For the carrierfrequency offset estimator, the receiver 700 can determine correlationsbetween two adjacent OFDM training symbols, and can then compute finalfrequency offset estimate, based on the coherently combinedcorrelations. In an implementation, the receiver 700 can be configuredto apply a maximum likelihood estimator (MLE) to estimate the frequencyoffset. Example operations can include taking the DFT of the correlatedfrequency domain training symbols, interpolation, and peak searching.

FIG. 8 illustrates a simplified functional block diagram of an exampleZFS OFDM satellite communication system 800 in which various methods andsystems for ZFS OFDM transmission and robust ZFS OFDM synchronization,and aspects thereof, can be implemented. The ZFS OFDM satellitecommunication system 800 can include a content host server 802configured to interface to a wide area network (WAN) 804, e.g., theInternet, for functions that can include transmitting content, forexample from a content storage 806, through the WAN 804, to a satellitegateway server 808. The satellite gateway server 808 can performsatellite uplink protocol operations and provide a resulting satelliteprotocol compliant packet stream to the ZFS OFDM satellite uplinktransmission unit 810. An example implementation of the ZFS OFDMsatellite uplink transmission unit can include the FIG. 1 system 100,with the packet stream output of the FIG. 8 satellite gateway server 808being the “data in” of FIG. 1. The ZFS OFDM satellite uplinktransmission unit 810 can output, for example, through a transmissiondish 812, a corresponding ZFS OFDM satellite uplink signal 814 to asatellite 816. The satellite 816 can be, for example, a space-borne HighThroughput Satellite (HTS) configured to transmit data to a plurality ofnarrowly focused regional spot beams. The satellite 816 can in turn, ina bent-pipe satellite communication configuration, transmit acorresponding ZFS OFDM satellite downlink signal 818 to a plurality ofnarrowly focused regional spot beams. A portion of one of the spotbeams, labeled “SB,” is visible in FIG. 8. A VSAT terminal receiver (ortransceiver) 820 can be located in the spot beam SB, and can include aVSAT ZFS robust synchronized receiver 822 that can be configured, forexample, according to the FIG. 2 ZFS robust synchronized receiver 200.The VSAT ZFS robust synchronized receiver 822 can connect, for example,to a WiFi station (not visible in FIG. 8) for wireless link to a laptop824 or equivalent user device. The VSAT terminal receiver can beconfigured to provide an uplink 826, e.g., a control uplink, to thesatellite 816, for communication as a downlink 828 to the satellitegateway server 808.

In some implementations, more than one satellite may be used, or othertypes of satellites may be used, including, but not limited to, FixedSatellite Service (FSS) High Throughput Satellite (HTS). In someimplementations, satellite 816 can be configured to receive data fromone or more gateway stations for retransmission via spot beams to remotesites located within cells defined by the spot beams.

In one various supplemental or alternative implementations, an elevatedplatform other than a satellite can be used to receive a logicalequivalent to the OFDM satellite uplink signal 814 and transmit, to aZFS robust synchronization OFDM receiver implemented as other than aVSAT, a logical equivalent to the ZFS OFDM satellite downlink signal818. Examples of an elevated platform can include, and are not limitedto, a balloon, airship, or unmanned aircraft vehicle (UAV), supportingtransponder equipment such as provided in an HTS.

FIG. 9 is a block diagram illustrating a computer system 900 upon whichaspects of this disclosure may be implemented, such as, but not limitedto, particular logic blocks and specific engines described in referenceto FIG. 2, and processes described at FIGS. 3, 4, 5, and 6. It will beunderstood that logic blocks illustrated in FIG. 9 represent functions,and do not necessarily correspond to particular hardware on a one-to-onebasis.

Referring to FIG. 9, the computer system 900 can include a dataprocessor 902, instruction memory 904, and a general purpose memory 906,coupled by a bus 908.

The instruction memory 906 can include a tangible medium retrievablystoring computer-readable instructions, labeled as “first module” 910,that when executed by the data processor 902 cause the processor toperform operations, such as described for the FIG. 2 correlation engine208, and computer-readable instructions, labeled as “second module” 912,that when executed by the data processor 902 cause the processor toperform operations, such as described for the FIG. 2 timing andcorrelation metric engine 210. The instruction memory 906 can store, onthe tangible medium, computer-readable instructions, labeled as “thirdmodule” 914 and “fourth module” 916, that when executed by the dataprocessor 902 cause the processor to perform operations such asdescribed for the FIG. 2 ETO engine 220, and frequency offset engine222, respectively.

The computer system 900 can also include a communications interface 918,configured to interface with a local network 920 for accessing a localserver 922, and to communicate through an Internet service provider(ISP) 924 to the Internet 926, and access a remote server 928. Thecomputer system 900 can also include a display 930 and a user interface932, such as a touchscreen or keypad.

The term “machine-readable medium” as used herein refers to any mediumthat participates in providing data that causes a machine to operationin a specific fashion. Forms of machine-readable media include, forexample, a floppy disk, a flexible disk, hard disk, magnetic tape, orany other magnetic medium, a CD-ROM, any other optical medium,punchcards, papertape, any other physical medium with patterns of holes,a RAM, a PROM, and EPROM, a FLASH-EPROM, any other memory chip orcartridge, a carrier wave as described hereinafter, or any other mediumfrom which a computer can read.

While the foregoing has described what are considered to be the bestmode and/or other examples, it is understood that various modificationsmay be made therein and that the subject matter disclosed herein may beimplemented in various forms and examples, and that the teachings may beapplied in numerous applications, only some of which have been describedherein. It is intended by the following claims to claim any and allapplications, modifications and variations that fall within the truescope of the present teachings.

Unless otherwise stated, all measurements, values, ratings, positions,magnitudes, sizes, and other specifications that are set forth in thisspecification, including in the claims that follow, are approximate.They are intended to have a reasonable range that is consistent with thefunctions to which they relate and with what is customary in the art towhich they pertain.

The scope of protection is limited solely by the claims that now follow.That scope is intended and should be interpreted to be as broad as isconsistent with the ordinary meaning of the language that is used in theclaims when interpreted in light of this specification and theprosecution history that follows and to encompass all structural andfunctional equivalents. Notwithstanding, none of the claims are intendedto embrace subject matter that fails to satisfy the requirement ofSections 101, 102, or 103 of the Patent Act, nor should they beinterpreted in such a way. Any unintended embracing of such subjectmatter is hereby disclaimed.

Except as stated immediately above, nothing stated or illustrated hereinis intended or should be interpreted to cause a dedication of anycomponent, step, feature, object, benefit, advantage, or equivalent tothe public, regardless of whether it is or is not recited in the claims.

It will be understood that terms and expressions used herein have theordinary meaning accorded to such terms and expressions in theirrespective areas of inquiry and study except where specific meaningshave otherwise been set forth herein. Relational terms such as first andsecond and the like may be used solely to distinguish one entity oraction from another without necessarily requiring or implying any actualsuch relationship or order between such entities or actions. The terms“comprises,” “comprising,” and any other variation thereof, are intendedto cover a non-exclusive inclusion, such that a process, method,article, or apparatus that comprises a list of elements does not includeonly those elements but may include other elements not expressly listedor inherent to such process, method, article, or apparatus. An elementpreceded by “a” or “an” does not, without further constraints, precludethe existence of additional identical elements in the process, method,or apparatus comprising the element.

The Abstract of the Disclosure is provided to allow the reader toquickly identify aspects of the disclosed subject matter. In theforegoing Detailed Description, it can be seen that various features aregrouped together in various examples for the purpose of streamlining thedisclosure. This method of disclosure is not to be interpreted asreflecting an intention that any claim requires more features than theclaim expressly recites. Rather, as the following claims reflect,inventive subject matter lies in less than all features of a singledisclosed example. Thus the following claims are hereby incorporatedinto the Detailed Description, with each claim standing on its own as aseparately claimed subject matter.

What is claimed is:
 1. A system for synchronizing a receiving of an orthogonal frequency division multiplexing (OFDM), the system comprising: an input data interface configured to receive a sequence of signal samples and associated indexes; a processing unit; and a memory configured to a store a plurality of instructions that when read and executed by the processing unit cause the processing unit to: generate an OFDM symbol frame, add a training symbol prefix to the OFDM symbol frame, the training symbol prefix including a plurality of training symbols, each of the training symbols including N sub-symbol fields, wherein a first N/2 of the sub-symbol fields are set at a zero value, a second N/2 of the sub-symbol fields carry corresponding symbols of a N/2 sub-symbol pseudo random training symbol, and a first half of the N/2 sub-symbol pseudo random training symbol is symmetrical to a second half of the N/2 sub-symbol pseudo random training symbol, and transmit the training symbol prefix as a multi-carrier transmission within an OFDM N-sub-carrier transmission, the transmit being configured such that: the multi-carrier prefix transmission carries signal power on a first N/2 of the N sub-carriers and suppresses signal power on a second N/2 of the sub-carriers, and the first N/2 sun-carriers and the second N/2 sub-carriers are respectively aligned at alternating positions in the frequency domain.
 2. The system of claim 1, wherein the adding of the training symbol prefix to the OFDM symbol frame is configured wherein: the N sub-symbol fields include N/2 even sub-symbol fields interleaved with N/2 odd sub-symbol fields, the N/2 even sub-symbol fields are the first N/2 of the sub-symbol fields, and the N/2 odd sub-symbol fields are the second N/2 of the sub-symbol fields.
 3. The system of claim 1, wherein to add the training symbol prefix to the OFDM symbol frame is configured wherein: the N sub-symbol fields include N/2 even sub-symbol fields interleaved with N/2 odd sub-symbol fields, the N/2 odd sub-symbol fields are the first N/2 of the sub-symbol fields, and the N/2 even sub-symbol fields are the second N/2 of the sub-symbol fields.
 4. The system of claim 1, wherein: to add the training symbol prefix to the OFDM symbol frame is configured such that each of the N/2 sub-symbols of the pseudo random training symbol is a complex value, the complex value being a mapping of a corresponding information symbol to an X-Y position in a complex plane, “X” being an in-phase component and “Y” being a quadrature component; and to transmit the training symbol prefix as the multi-carrier transmission within the OFDM is further configured such that the signal power of each of the first N/2 of the N sub-carriers is a modulation, in amplitude and phase, of the sub-carrier N/2 corresponding to the complex value of one of the N/2 sub-symbols of the pseudo random training symbol.
 5. The system of claim 1, such that for each of the training symbols: a first half of the training symbol's N sub-symbol fields includes a first half of the N/2 sub-symbols of the pseudo random training symbol interleaved with a first half of the N/2 sub-symbol fields of the training symbol that are set at a zero value, a second half of the training symbol's N sub-symbol fields includes a second half of the N/2 sub-symbols of the pseudo random training symbol interleaved with a second half of the N/2 sub-symbol fields of the training symbol that are set at a zero value, the first half of the N/2 sub-symbols of the pseudo random training symbol that is included in first half of the training symbol's N sub-symbol fields is identical to the second half of the N/2 sub-symbols of the pseudo random training symbol that is included in second half of the training symbol's N sub-symbol fields.
 6. The system of claim 5, such that for each of the training symbols the first half of the training symbol's N sub-symbol fields is symmetrical to the second half of the training symbol's N sub-symbol fields.
 7. A method for communicating a robust synchronization orthogonal frequency division multiplexing (OFDM) frame, comprising: generating an OFDM symbol frame; adding a training symbol prefix to the OFDM symbol frame, the training symbol prefix including a plurality of training symbols, each of the training symbols including N sub-symbol fields, wherein a first N/2 of the sub-symbol fields are set at a zero value, a second of the N/2 sub-symbol fields carry corresponding symbols of a N/2 sub-symbol pseudo random training symbol, and a first half of the pseudo random training symbol is symmetrical to a second half of the pseudo random training symbol; and transmitting the training symbol prefix as a multi-carrier transmission within an OFDM N-sub-carrier transmission, wherein the multi-carrier prefix transmission carries signal power on a first N/2 of the N sub-carriers and suppresses signal power on a second N/2 of the sub-carriers, and the first N/2 sub-carriers and the second N/2 sub-carriers are respectively aligned at alternating positions in the frequency domain.
 8. The method of claim 7, wherein the adding of the training symbol prefix to the OFDM symbol frame is configured such that: the N sub-symbol fields include N/2 even sub-symbol fields interleaved with N/2 odd sub-symbol fields, the N/2 even sub-symbol fields are the first N/2 of the sub-symbol fields, and the N/2 odd sub-symbol fields are the second N/2 of the sub-symbol fields.
 9. The method of claim 7, wherein adding the training symbol prefix to the OFDM symbol frame is configured wherein: the N sub-symbol fields include N/2 even sub-symbol fields interleaved with N/2 odd sub-symbol fields, the N/2 odd sub-symbol fields are the first N/2 of the sub-symbol fields, and the N/2 even sub-symbol fields are the second N/2 of the sub-symbol fields.
 10. The method of claim 7, wherein: adding the training symbol prefix to the OFDM symbol frame is configured such that each of the N/2 sub-symbols of the pseudo random training symbol is a complex value, the complex value being a mapping of a corresponding information symbol to an X-Y position in a complex plane, “X” being an in-phase component and “Y” being a quadrature component; and transmitting the training symbol prefix as the multi-carrier transmission within the OFDM is further configured such that the signal power of each of the first N/2 of the N sub-carriers is a modulation, in amplitude and phase, of the sub-carrier N/2 corresponding to the complex value of one of the N/2 sub-symbols of the pseudo random training symbol.
 11. The method of claim 7, wherein adding the training symbol prefix to the OFDM symbol frame is configured wherein, for each of the training symbols: a first half of the training symbol's N sub-symbol fields includes a first half of the N/2 sub-symbols of the pseudo random training symbol interleaved with a first half of the N/2 sub-symbol fields of the training symbol that are set at a zero value, a second half of the training symbol's N sub-symbol fields includes a second half of the N/2 sub-symbols of the pseudo random training symbol interleaved with a second half of the N/2 sub-symbol fields of the training symbol that are set at a zero value, the first half of the N/2 sub-symbols of the pseudo random training symbol that is included in first half of the training symbol's N sub-symbol fields is identical to the second half of the N/2 sub-symbols of the pseudo random training symbol that is included in second half of the training symbol's N sub-symbol fields.
 12. The method of claim 11, wherein adding the training symbol prefix to the OFDM symbol frame is further configured such that for each of the training symbols the first half of the training symbol's N sub-symbol fields is symmetrical to the second half of the training symbol's N sub-symbol fields.
 13. The method of claim 7, the method further including: receiving the OFDM N-sub-carrier transmission; sampling the received OFDM N-sub-carrier transmission and generating a corresponding sequence of signal samples and associated indexes; detecting a timing metric at each of a succession of the indexes and determining, as a first peak, an index at which the timing metric is at a local maximum; detecting the timing metric at indexes within a first region of the indexes and determining, as a second peak, an index at which the timing metric is at another local maximum, the first region being based in part on the first peak; selecting a coarse offset as the smaller of the first peak and the second peak; detecting a correlation metric at a plurality of indexes within a second region of the indexes, the second region being based at least in part on the coarse offset; and determining, as an estimated time offset, an index at which the correlation metric is a local maximum.
 14. The method of claim 7, wherein detecting the correlation metric at each of the plurality of indexes within the second region of the indexes includes, for each index: calculating a single-frame correlation metric for each frame among multiple frames with the succession of frames, to form multiple single-frame correlation metrics for the index, and combining the multiple single frame correlation metrics for the index into an accumulated correlation metric, and detecting the correlation metric for the index as the accumulated correlation metric.
 15. The method of claim 14, wherein combining the multiple single frame correlation metrics for the index into the accumulated correlation metric includes: selecting, as a selected combining, between a coherent combining and a non-coherent combining, and applying the selected combining.
 16. A system for synchronizing a receiving of an orthogonal frequency division multiplexing (OFDM), the system comprising: an OFDM transmitter, configured to transmit a succession of OFDM information frames, each of the OFDM information frames carrying, on N sub-carriers, a block of OFDM symbols; and a training symbol insertion unit configured to insert training symbol prefixes between successive OFDM information frames, each training symbol prefix including a plurality of training symbols, each of the training symbols including N sub-symbol fields, the training symbol insertion unit being further configured to set a first N/2 of the sub-symbol fields to a zero value, form a second N/2 of the sub-symbol fields to carry corresponding symbols of a N/2 sub-symbol pseudo random training symbol, and form a first half of the N/2 sub-symbol pseudo random training symbol to be symmetrical to a second half of the N/2 sub-symbol pseudo random training symbol, wherein the OFDM transmitter is further configured to: transmit the training symbol prefix as a multi-carrier transmission within an OFDM N-sub-carrier transmission, wherein: the multi-carrier prefix transmission carries signal power on a first N/2 of the N sub-carriers and suppresses signal power on a second N/2 of the sub-carriers, and the first N/2 sun-carriers and the second N/2 sub-carriers are respectively aligned at alternating positions in the frequency domain.
 17. The system of claim 16, wherein the training symbol insertion unit is further configured such that: the N sub-symbol fields include N/2 even sub-symbol fields interleaved with N/2 odd sub-symbol fields, the N/2 even sub-symbol fields are the first N/2 of the sub-symbol fields, and the N/2 odd sub-symbol fields are the second N/2 of the sub-symbol fields.
 18. The system of claim 16, wherein the training symbol insertion unit is further configured such that: the N sub-symbol fields include N/2 even sub-symbol fields interleaved with N/2 odd sub-symbol fields, the N/2 odd sub-symbol fields are the first N/2 of the sub-symbol fields, and the N/2 even sub-symbol fields are the second N/2 of the sub-symbol fields.
 19. The system of claim 16, wherein the training symbol insertion unit is further configured such that each of the N/2 sub-symbols of the pseudo random training symbol is a complex value having an in-phase component and a quadrature component; and the OFDM transmitter is further configured such that the signal power of each of the first N/2 of the N sub-carriers is a modulation, in amplitude and phase, of the sub-carrier N/2 corresponding to the complex value of one of the N/2 sub-symbols of the pseudo random training symbol.
 20. The system of claim 16, wherein the training symbol insertion unit is further configured such that a first half of the training symbol's N sub-symbol fields includes a first half of the N/2 sub-symbols of the pseudo random training symbol interleaved with a first half of the N/2 sub-symbol fields of the training symbol that are set at a zero value, a second half of the training symbol's N sub-symbol fields includes a second half of the N/2 sub-symbols of the pseudo random training symbol interleaved with a second half of the N/2 sub-symbol fields of the training symbol that are set at a zero value, the first half of the N/2 sub-symbols of the pseudo random training symbol that is included in first half of the training symbol's N sub-symbol fields is identical to the second half of the N/2 sub-symbols of the pseudo random training symbol that is included in second half of the training symbol's N sub-symbol fields, and for each of training symbol the first half of the training symbol's N sub-symbol fields is symmetrical to the second half of the training symbol's N sub-symbol fields. 